Electrochemical fuel cell assemblies are known for their ability to produce electricity and a subsequent reaction product through the interaction of a fuel being provided to an anode and an oxidant being provided to a cathode, thereby generating a potential between these electrodes. Such fuel cell assemblies are very useful and sought after due to their high efficiency, as compared to internal combustion fuel systems and the like. Fuel cell assemblies are additionally advantageous due to the environmentally friendly chemical reaction by-products that are produced, such as water. In order to control the temperature within the fuel cell assembly, a coolant is provided to circulate about a fuel cell assembly, usually water. This concentration and use of water within fuel cell assemblies makes them particularly sensitive to freezing temperatures.
Electrochemical fuel cell assemblies typically employ hydrogen as the fuel and oxygen as an oxidant where, as noted above, the reaction by-product is water. Such fuel cell assemblies may employ a membrane consisting of a solid polymer electrolyte, or ion exchange membrane, disposed between the two electrodes. The electrodes are usually supported by a porous, electrically conductive sheet material--typically carbon fiber paper. The ion exchange membrane is also known as a proton exchange membrane (hereinafter PEM), such as sold by DuPont under the trade name NAFION.TM., and usually has a catalyst layer formed thereon to provide a membrane-electrode interface so as to promote the desired electrochemical reaction. The membrane electrode assemblies are then electrically coupled in order to provide a path for conducting electrons between the electrodes when an external load is applied.
In operation, hydrogen fuel permeates the porous electrode support material of the anode and reacts at the catalyst layer to form hydrogen ions and electrons. The hydrogen ions migrate through the membrane to the cathode and the electrons flow through an external circuit to the cathode. At the cathode, the oxygen-containing gas supply also permeates through the porous electrode support material and reacts with the hydrogen ions and the electrons from the anode at the catalyst layer to form the by-product water. Not only does the ion exchange membrane facilitate the migration of these hydrogen ions from the anode to the cathode, but the ion exchange membrane also acts to isolate the hydrogen fuel from the oxygen-containing gas oxidant. The reactions taking place at the anode and cathode catalyst layers are represented by the equations: EQU Anode reaction: H.sub.2.fwdarw.2H.sup.+ +2e EQU Cathode reaction: 1/2O.sub.2 +2H.sup.+ +2e .fwdarw.H.sub.2 O
Conventional fuels cells may have the ion exchange membrane positioned between two gas-permeable, electrically conductive plates, referred to as the anode and cathode plates. The plates are typically formed from graphite, a graphite composite, or the like. The plates act as a structural support for the two porous, electrically conductive electrodes, as well as serving as current collectors and providing the means for carrying the fuel and oxidant to the anode and cathode, respectively. They are also utilized for carrying away the reactant by-product water during operation of the fuel cell.
When flow channels are formed within these plates for the purposes of circulating either fuel or oxidant in the anode and cathode plates, they are referred to as fluid flow field plates or water transfer plates. When these plates simply overlay channels formed in the anode and cathode porous material, they are referred to as separator plates. The plates may have formed therein reactant feed manifolds which are utilized for supplying fuel to the anode flow channels or, alternatively, oxidant to the cathode flow channels. They also have corresponding exhaust manifolds to direct unreacted components of the fuel and oxidant streams, and any water generated as a by-product, from the fuel cell. Additionally, the fluid flow field plates or water transfer plates may support channels for the purpose of circulating a coolant about the fuel cell, as well as selectively supporting coolant manifolds.
Recent efforts at producing the fuel for fuel cell assemblies have focused on utilizing impure hydrogen produced from the chemical conversion of hydrocarbon fuels, such as methane, natural gas, gasoline or the like, into hydrogen. This process requires that the hydrogen produced must be efficiently converted to be as pure as possible, thereby ensuring that a minimal amount of carbon monoxide and other undesirable chemical byproducts are produced. For PEM type fuel cell assemblies this conversion of hydrocarbons is generally accomplished through the use of a steam reformer, a shift converter and a selective oxidizer in combination.
The catalyst layer in a fuel cell assembly is typically a carbon supported platinum or platinum alloy, although other noble metals or noble metal alloys may be utilized. Multiple electrically connected fuel cell assemblies consisting of two or more anode plate/membrane/cathode plate combinations are referred to as a fuel cell stack. The fuel cells in a fuel cell stack are typically electrically connected in series.
It is necessary to provide some heat exchange system whereby the excess heat generated during the reaction process is extracted so as not to adversely effect the fuel cell operation. As disclosed above, the anode and cathode plates provide coolant channels for the circulation of a water coolant, as well as the wicking and carrying away of excessive water produced as a by-product of the fuel cell assembly operation. The water so collected and circulated through a fuel cell assembly is susceptible to the freezing effects of temperatures below 32.degree. F. (0.degree. C.) and may therefore damage and impair the operation of the fuel cell assembly as the water expands when subjected to such temperatures.
With the forgoing problems and concerns in mind, it is the general object of the present invention to provide for the thermal management of a fuel cell assembly which overcomes the above-described drawbacks, as well as to affirmatively maximize the efficiency of the fuel cell even in times of freezing temperatures.